1. Field of the Invention
This invention relates to electronic watthour meter circuits, and more particularly, to an electronic watthour meter circuit having a digital counter arrangement for computing the product of voltage and current components.
2. Description of the Prior Art
The electromechanical rotating disc type of watthour meter continues to be used almost exclusively in utility and commerical electrical energy metering applications, but alternative methods are sought in developing electronic watthour meter circuits with high reliability and low cost. Basically two methods are known to measure electrical energy by electronic watthour meters. The electrical energy supplied through a circuit is the product of the measured power and the period of time that that power level is measured. AC electric power can be expressed as a mathematical equation as either: ##EQU1## WHERE
V = INSTANTANEOUS VOLTAGE;
I = INSTANTANEOUS CURRENT;
T = a predetermined time interval.
Or where
V = maximum voltage;
I = maximum current;
.theta. = PHASE ANGLE BETWEEN VOLTAGE AND CURRENT; AND
COS .theta. = POWER FACTOR.
Electronic watthour meters that utilize the integration method must sample the voltage and current waveforms continuously. They also provide some form of function multiplier and integrator. In U.S. Pat. No. 3,500,200 a circuit is described for modulating the height and width of pulses of a pulse train responsive to voltage and current signals. The area under each pulse then represents power. An average of the pulse areas over measured time intervals gives the average power supplied through an external circuit. U.S. Pat. Nos. 3,760,273 and 3,875,509 disclose similar methods of pulse modulation and averaging in energy measuring circuits.
Peak signal level measuring electronic watthour meters typically include some form of function multiplier and a means for determining the power factor quantity cos .theta.. In U.S. Pat. No. 3,171,311 a phase comparator is disclosed to compute the phase angle .theta. and a cosine function circuit uses the angle .theta. to compute the power factor quantity cos .theta..